This paper gives a very brief introduction to the Semantic Wave Theory proposed by Karl Maton, a famous sociologist of Sydney University in Australia, with the focus on the two concepts of Semantic Gravity and Semantic Density and their impact on the formation and evolution of semantic waves. And then it tries to show, by data analysis, that teaching methods, such as definition, classification and exemplification can be used to reduce semantic density, as pointed out by scholars like Maton. Besides, it offers two points of view of its writer: 1) Source tracing, association, guiding, figurative memorization and paraphrasing also have their role to play in density reduction and knowledge accumulation; and 2) it is sometimes necessary to do things in an opposite way and increase semantic density in the process of teaching so as to help the students to accumulate more and more new knowledge.